Bearing grease composition

ABSTRACT

A bearing grease composition used under clean atmosphere, as in a bearing of hard disc drive, which has small dust generation (scatter), a long life, excellent acoustic performance and low torque.  
     It has been found that especially the bearing grease composition comprising a carbonate having a branched alkyl group of 13 to 15 carbon atoms and a metallic soap shows remarkable characteristics.

FIELD OF THE INVENTION

[0001] The present invention relates to a bearing grease composition forhard disc drive or the like. More particularly, the invention relates toa bearing grease composition for spindle motors which are used underclean environment, as in memories such as hard disc drive (HDD) orfloppy disc drive (FDD) in computers.

BACKGROUND OF THE INVENTION

[0002] In general, performances required for a bearing greasecomposition used in memories, for examples, HDD or FDD in computers arethat dust generation (scatter) is low, torque is small, acousticperformance is excellent and life is long. In particular, in HDD usedunder a clean atmosphere, fine particles of gaseous oil or greasescattered from the inside of a bearing during revolution contaminatesthe surface of a disc, resulting in cause of wrong operation. Therefore,it is considered to be most important to suppress the amount of scatter.

[0003] A sodium complex soap-based grease comprising a mineral oil as abase oil or a lithium soap-based grease comprising as a base oil, asynthetic ester (diester oil or polyol ester oil) which is a reactionproduct of an organic acid and an alcohol has conventionally been usedas HDD bearing grease.

[0004] The sodium complex soap-based grease comprising a mineral oil asa base oil has been used for a long time with appreciation that theamount of scatter is small. However, the grease had the problems thatdispersion of a thickening agent in the grease is poor, and it isdifficult to form a homogeneous mixture, so that acoustic and vibratingperformances during evolution of the bearing is not good; moistureabsorption is high, and the grease cures with the passage of time, sothat flowability of the grease in the bearing becomes poor, causingdefective lubrication.

[0005] Further, the lithium soap-based grease comprising as a base oil,a synthetic ester (diester or polyol ester oil) which is a reactionproduct of an organic acid and an alcohol had no problem due to thatdispersibility of the lithium soap is good, and low torque property wasgood.

[0006] However, the lithium soap-based grease tends to scatter,and ifthe grease is used as it is, there is the great possibility to damage ahard disc. Therefore, in order to prevent the damage, the grease is usedin combination with an expensive magnetic fluid seal which is used in amotor. This rather results in increase of cost of a motor and preventsthe motor from being miniaturized. Further, for example, due to thedemand of high speed revolution and high precision of the motor, thisgrease does not sufficiently withstand acoustic performance and torque,disorder has sometimes occurred in acoustic performance or torque.

SUMMARY OF THE INVENTION

[0007] As a result of extensive investigations to overcome theabove-described problems in the prior art, it has been found thatproperties required for a bearing grease composition which is used undera clean environment as in, for example, memories such as HDD or FDD aresatisfied by using a metallic soap-based grease comprising as a base oila carbonate containing an organic carbonate represented by the formula(I) described hereinafter. The present invention has been completedbased on this finding.

[0008] Accordingly, an object of the present invention is to provide agrease composition which can maintain stable low torque property, lownoise property and low scatter property even at high speed revolution.

[0009] According to the present invention, there is provided a bearinggrease composition comprising:

[0010] (a) a carbonate compound represented by the following formula (I)

[0011] wherein R and R′, which may be the same or different, eachrepresent a branched alkyl group having 13 to 15 carbon atoms; and

[0012] (b) a group consisting of an alkali metal salt and/or an alkalineearth metal salt, which are synthesized from a hydroxide of an alkalimetal or an alkaline earth metal and a higher hydroxyfatty acid having10 or more carbon atoms or a higher hydroxyfatty acid having at leastone hydroxyl group and having 10 or more carbon atoms.

[0013] The branched alkyl group R and R′ in the carbonate compoundrepresented by the formula (I) is represented by the following formula(II):

[0014] wherein n is a number of 13 to 15 and m is a number of 0 to 6.

[0015] The representative examples of the branched alkyl group aregroups represented by the following formulae (III), (IV) and (V):

[0016] The bearing grease composition of the present inventionpreferably comprises 70 to 95 parts by weight of component (a) of thecarbonate compound, and 5 to 30 parts by weight of component (b) of thealkali metal salt and/or alkaline earth metal salt.

[0017] If the amount of component (b) of the alkali metal salt and/oralkaline earth metal salt is less than 5 parts by weight, a workedpenetration becomes soft, and as a result, the grease composition leaksor scatters during revolution of a bearing, resulting in possibility ofcontaminating HDD, FDD and the like.

[0018] On the other hand, if the above amount of component (b) exceeds30 parts by weight, the grease composition becomes too hard, andflowability of grease in the bearing becomes poor. As a result, there isthe possibility of causing defective lubrication.

[0019] The bearing grease composition according to the present inventioncan contain, in addition to the components (a) and (b) described above,lubricants other than the component (a), and various additives, as athird component.

[0020] Examples of the lubricants used in the present invention includemineral oils, synthesized hydrocarbon oils, ether oils and ester oils,which are generally used as a base oil of grease compositions.

[0021] The additives used in the present invention are additivesgenerally used in grease composition, and the examples thereof includeantioxidants and rust inhibitors.

[0022] The grease composition of the present invention may furthercontain one or more of thickening agents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The present invention described in detail below.

[0024] (1) According to the preferred embodiments of the presentinvention, a bearing grease composition preferably comprises:

[0025] (a) a carbonate compound represented by the following formula (I)

[0026]  wherein R and R′ which may be the same or different eachrepresent a branched alkyl group having 13 to 15 carbon atoms; and

[0027] (b) a group consisting of an alkali metal salt and/or an alkalineearth metal salt, synthesized from a hydroxide of an alkali metal or analkaline earth metal and a higher hydroxyfatty acid having 10 or morecarbon atoms or a higher hydroxyfatty acid having at least one hydroxylgroup and having 10 or more carbon atoms.

[0028] (2) Preferably, the bearing grease composition comprises:

[0029] (a) a carbonate compound represented by the following formula (I)

[0030]  wherein R and R′, which may be the same or different, eachrepresent a branched alkyl group having 13 to 15 carbon atoms, whereinthe branched alkyl group R and R′ in the carbonate compound representedby the formula (I) is represented by the following formula (II):

[0031]  wherein n is a number of 13 to 15 and m is a number of 0 to 6;and

[0032] (b) a group consisting of an alkali metal salt and/or an alkalineearth metal salt, synthesized from a hydroxide of an alkali metal or analkaline earth metal and a higher hydroxyfatty acid having 10 or morecarbon atoms or a higher hydroxyfatty acid having at least one hydroxylgroup and having 10 or more carbon atoms.

[0033] (3) The bearing grease composition comprises 70 to 95 parts byweight of the carbonate compound represented by formulas (I) and (II)and 5 to 30 parts by weight of the member selected from the groupconsisting of an alkali metal salt and/or an alkaline earth metal salt,synthesized from a hydroxide of an alkali metal or an alkaline earthmetal and a higher hydroxyfatty acid having 10 or more carbon atoms or ahigher hydroxyfatty acid having at least one hydroxyl group and having10 or more carbon atoms.

[0034] (4) The bearing grease composition for hard disc drive includespreferably a plurality of carbonate compounds.

[0035] (5) The bearing grease composition for hard disc drive in whichthe carbonate compound preferably has a dynamic viscosity of 10 to 50mm²/sec at 40° C.

[0036] (6) Preferably, the bearing grease composition further comprisesone or more thickening agents.

[0037] (7) The bearing grease composition as in any of theabove-described aspects (1) to (6) comprising further lubricants and/oradditives.

[0038] The carbonate compound can be used alone or as mixtures thereof.

[0039] The present invention will now be described in more detail belowby referring to the following examples, but the invention should not belimited thereto.

EXAMPLES 1 TO 6

[0040] Those examples are the preparation example of the typical bearinggrease composition consisting of component (a) and component (b).

[0041] A carbonate represented by the following formula (VI):

[0042] that is, in the formula (I), R is the branched alkyl grouprepresented by the following formula (IV):

[0043] and R′ is the branched alkyl group represented by the followingformula (V):

[0044] (this carbonate is hereinafter referred to as“carbonate oil A”)alone, or a mixed oil of a carbonate containing commercially availableorganic carbonate having different structure of an alkyl group (CYCRANTT-394, trade name, a product of Mitsui Petrochemical Industries, Ltd.)(this ester is hereinafter referred to as“carbonate ester oil B”) andthe carbonate ester oil A was mixed with lithium stearate and/or lithium12 hydroxystearate in the proportions as shown in Table 1 such that thesum of those was 100 mass %. The resulting mixture was heated to220-230° C. while stirring until the whole of mixture became liquid. Theliquid thus obtained was poured into a stainless steel vessel at a depthof 3-5 mm. The liquid was cooled at 50° C. or less and then homogenizedwith three rollers to obtain a grease composition.

EXAMPLES 7 AND 8

[0045] Grease compositions were obtained in the same manner as inExamples 1 to 6 above except that an alkyl diphenyl ether or a polyolester was further used as a third component other than components (a)and (b) in the proportion as shown in Table 1.

COMPARATIVE EXAMPLES 1 AND 2

[0046] Two kinds of commercially available greases in which a base oiland a thickening agent are known were used for the sake of comparison.In Table 2, reference sign+means that such a compound is contained.

COMPARATIVE EXAMPLE 3 AND 4

[0047] Grease compositions were obtained in the same manner as inExamples 1-6 except that a base oil and a lithium soap were used in theproportions as shown in Table 2.

[0048] The grease compositions obtained in the above Examples andComparative Examples were measured for a worked penetration and adropping point, and were subjected to motor characteristic test, underthe same conditions. The results obtained are shown in Tables 1 and 2.

[0049] The worked penetration was measured according to JIS K2220, 5.3,and the dropping point was measured according to JIS K2220, 5.4.

[0050] The motor characteristic test was conducted such that a bearinghaving included therein a grease composition to be tested wasincorporated in a spindle motor, the motor was rotated at 10,000 rpm ata normal temperature, and noise generated, amount of evaporation(scatter) and rotating torque were measured.

[0051] Noise was measured with a microphone at a place which is 30 cmapart from a hub edge of the motor during rotation of motor.

[0052] The amount of evaporation (scatter) was determined by thedifference between weight of motor before rotation and weight of motorafter rotation.

[0053] The torque was determined by measuring electric current value atthe rotation of motor with an ammeter, and the torque stability wasdetermined by the difference between the maximum electric current valueand the minimum electric current value.

[0054] The judgement results according to performances required for abearing grease composition for HDD or the like on each evaluation itemare shown in Table 1 (Examples) and Table 2 (Comparative Examples).

[0055] The smaller the value, the better the low noise property; thesmaller the amount of evaporation (scatter), the better the low scatterproperty; the lower the value, the better the low torque property; andthe smaller the fluctuation, the better the torque stability.

[0056] The tests were evaluated with the following four grades. TABLE 1Example 1 2 3 4 5 6 7 8 Thickening agent StLi 25 25 10 15 20 20 120HSt-Li 15 10 5 Base oil Carbonate A 75 85 38 90 90 80 70 70 Carbonate B37 ADE 10 POE 10 Viscosity of 18 18 50 18 18 18 25 21 base oil (40° C.)Worked 198 203 196 305 247 181 210 205 Penetration (25° C.) Droppingpoint (° C.) 197 194 198 193 193 195 197 195 Motor characteristic TestLow noise property A A B B B A B B Low scatter property A B A B B B A ALow torque property A A B A A A B B Torque stability A A B B B A A ATotal evaluation A A B B B A B B

[0057] TABLE 1 Example 1 2 3 4 5 6 7 8 Thickening agent StLi 25 25 10 1520 20 12OH St-Li 15 10  5 Base oil Carbonate A 75 85 38 90 90 80 70 70Carbonate B 37 ADE 10 POE 10 Viscosity of 18 18 50 18 18 18 25 21 baseoil (40° C.) Worked 198  203  196  305  247  181  210  205  Penetration(25° C.) Dropping point 197  194  198  193  193  195  197  195  (° C.)Motor characteristic Test Low noise A A B B B A B B property Low scatterA B A B B B A A property Low torque A A B A A A B B property Torquestability A A B B B A A A Total A A B B B A B B evaluation

[0058] TABLE 2 COMPARATIVE EXAMPLE 1 2 3 4 Thickening agent StLi +  25 25 12OH St-Li + Na-Complex + Base oil Carbonate ester A Carbonate esterB  75 Diester oil +  38 POE +  37 Mineral oil + Viscosity of base oil(40° C.) 26   145  18 130 Worked penetration 250   205 197 181 Droppingpoint (° C.) 194 >260 197 199 Motor characteristic test Low noiseproperty C D D B Low scatter property B A D A Torque C B B D Torquestability D C D C Total evaluation C C D C

[0059] As shown in Table 1 above, the bearing grease composition of thepresent invention shows low evaporation (scatter), low noise, low torqueproperty and stable torque.

What is claimed is:
 1. A bearing grease composition comprising: (a) acarbonate compound represented by the following formula (I)

wherein R and R′, which may be the same or different, each represent abranched alkyl group having 13 to 15 carbon atoms; and (b) a groupconsisting of an alkali metal salt and/or an alkaline earth metal salt,synthesized from a hydroxide of an alkali metal or an alkaline earthmetal and a higher hydroxyfatty acid having 10 or more carbon atoms or ahigher hydroxyfatty acid having at least one hydroxyl group and having10 or more carbon atoms.
 2. The bearing grease composition as claimed inclaim 1, wherein the branched alkyl group R and R′ in the carbonatecompound represented by the formula (I) is represented by the followingformula (II):

wherein n is a number of 13 to 15 and m is a number of 0 to
 6. 3. Thebearing grease composition as claimed in claim 1, which comprises 70 to95 parts by weight of the component (a) and 5 to 30 parts by weight ofthe component (b).
 4. The bearing grease composition as claimed in claim1, 2 or 3, wherein the component (a) includes a plurality of carbonatecompounds.
 5. The bearing grease composition as claimed in claim 1, 2, 3or 4, which further comprises one or more thickening agents.
 6. Thebearing grease composition as claimed in claim 1, 2, 3, 4 or 5, whichfurther comprises lubricants and/or additives.